Photoelectric conversion device, image capturing system, and method of manufacturing photoelectric conversion device

ABSTRACT

A photoelectric conversion device includes a photoelectric conversion unit which is arranged in a semiconductor substrate, a charge holding portion which is arranged in the semiconductor substrate and temporarily holds a charge generated by the photoelectric conversion unit, a first transfer electrode which is arranged at a position above the semiconductor substrate to transfer a charge generated by the photoelectric conversion unit to the charge holding portion, a charge-voltage converter which is arranged in the semiconductor substrate and converts a charge into a voltage, and a second transfer electrode which is arranged at a position above the semiconductor substrate to transfer a charge held by the charge holding portion to the charge-voltage converter, and the first transfer electrode is arranged to cover the charge holding portion, and not to overlap the second transfer electrode when viewed from a direction perpendicular to the upper surface of the semiconductor substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photoelectric conversion device,image capturing system, and method of manufacturing a photoelectricconversion device.

2. Description of the Related Art

An active-pixel type photoelectric conversion device represented by aCMOS image sensor includes a pixel array in which a plurality of pixelsare arranged in a matrix pattern, and an output circuit which processesa signal read out from the pixel array onto a signal line and outputsthe processed signal. Some photoelectric conversion devices of this typehave a global electronic shutter function in which all pixels in thepixel array start and end charge accumulation operations at the sametimings.

Note that the CMOS image sensor cannot read out signals of pixels of allrows in the pixel array onto signal lines at the same time in terms ofits structure. In a photoelectric conversion device having the globalelectronic shutter function, each pixel immediately transfers a chargeaccumulated by a photodiode (PD) to a carrier pocket (CP) in place of afloating diffusion (FD) at the end timing of the charge accumulationoperation. A charge which is transferred to and held by the CP in eachof pixels of each row is sequentially transferred from the CP to the FDfor each row, and a signal according to the charge transferred to the FDis read out onto a signal line by an amplification transistor of eachpixel.

Note that the CP has a role of holding a charge after completion ofaccumulation of a certain pixel until the charge is read out. Therefore,when light becomes incident on the CP and a charge is generated byphotoelectric conversion at a PN junction which contacts the CP, leaknoise of light is mixed in a signal held in the CP. As a result, theimage quality of an obtained image deteriorates.

In order to solve the above problem, in Japanese Patent Laid-Open No.2007-115803, the CP is covered by a first transfer gate electrode usedto transfer a charge from the PD to the CP, and the first transfer gateelectrode is covered by a second transfer gate electrode used totransfer a charge from the CP to the FD. Thus, according to JapanesePatent Laid-Open No. 2007-115803, intrusion of light into the CP can besuppressed.

With the technique described in Japanese Patent Laid-Open No.2007-115803, a pattern of the first transfer gate electrode is formedabove a semiconductor substrate, an insulation film is formed to coverthe first transfer gate electrode, and a pattern of the second transfergate electrode is then formed on that insulation film. With thismanufacturing method, processes required to form the first and secondtransfer gate electrodes are complicated.

SUMMARY OF THE INVENTION

The present invention provides a photoelectric conversion device, whichis advantageous to simplify a process for forming a structure requiredto optically shield a charge holding portion used to temporarily hold acharge generated by a photoelectric conversion unit.

The first aspect of the present invention provides a photoelectricconversion device comprising a photoelectric conversion unit which isarranged in a semiconductor substrate, a charge holding portion which isarranged in the semiconductor substrate and temporarily holds a chargegenerated by the photoelectric conversion unit, a first transferelectrode which is arranged at a position above the semiconductorsubstrate to transfer a charge generated by the photoelectric conversionunit to the charge holding portion, a charge-voltage converter which isarranged in the semiconductor substrate and converts a charge into avoltage, and a second transfer electrode which is arranged at a positionabove the semiconductor substrate to transfer a charge held by thecharge holding portion to the charge-voltage converter, wherein thefirst transfer electrode is arranged to cover the charge holding portionso as to optically shield the charge holding portion, and not to overlapthe second transfer electrode.

The second aspect of the present invention provides an image capturingsystem comprising a photoelectric conversion device according to thefirst aspect of the present invention, an optical system which forms animage on an imaging surface of the photoelectric conversion device, anda signal processing unit which generates image data by processing asignal output from the photoelectric conversion device.

The third aspect of the present invention provides a method ofmanufacturing a photoelectric conversion device having a semiconductorsubstrate in which a photoelectric conversion unit, a charge holdingportion which temporarily holds a charge generated by the photoelectricconversion unit, and a charge-voltage converter which converts a chargeinto a voltage are arranged, the method comprising the first step ofsimultaneously executing formation of a first polysilicon layer on thesemiconductor substrate, of a first transfer electrode used to transfera charge generated by the photoelectric conversion unit to the chargeholding portion, and formation of a second polysilicon layer on thesemiconductor substrate, of a second transfer electrode used to transfera charge of the charge holding portion to the charge-voltage converter,the second step of forming an insulation film to cover the firstpolysilicon layer and the second polysilicon layer, the third step ofetching the insulation film to expose an upper surface of the firstpolysilicon layer and an upper surface of the second polysilicon layer,the fourth step of forming a metal layer to cover the first polysiliconlayer, the second polysilicon layer, and the etched insulation film, andthe fifth step of performing annealing to respectively convert an upperportion of the first polysilicon layer and an upper portion of thesecond polysilicon layer into a first metal silicide layer and a secondmetal silicide layer, so as to simultaneously form the first transferelectrode including a lower portion of the first polysilicon layer andthe first metal silicide layer, and the second transfer electrodeincluding a lower portion of the second polysilicon layer and the secondmetal silicide layer, wherein in the first step, the first polysiliconlayer is formed to cover the charge holding portion and not to overlapthe second polysilicon layer, and in the fifth step, the first transferelectrode is formed to cover the charge holding portion so as tooptically shield the charge holding portion, and not to overlap thesecond transfer electrode.

According to the present invention, a photoelectric conversion device,which is advantageous to simplify a process for forming a structurerequired to optically shield a charge holding portion used totemporarily hold a charge generated by a photoelectric conversion unit,can be provided.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the structure of a photoelectricconversion device 100 according to the first embodiment of the presentinvention;

FIG. 2 is a circuit diagram showing the arrangement of a pixel accordingto the first embodiment of the present invention;

FIG. 3 is a view showing a layout arrangement example of thephotoelectric conversion device 100 according to the first embodiment ofthe present invention;

FIGS. 4A and 4B are sectional views showing the sectional structures ofthe photoelectric conversion device 100 according to the firstembodiment of the present invention;

FIG. 5 is a block diagram showing the arrangement of an image capturingsystem to which the photoelectric conversion device according to thefirst embodiment is applied;

FIG. 6 is a sectional view showing the sectional structure of aphotoelectric conversion device 200 according to the second embodimentof the present invention;

FIGS. 7A and 7B are views showing the structure of a photoelectricconversion device 300 according to the third embodiment of the presentinvention;

FIGS. 8A and 8B are views showing the structure of a photoelectricconversion device 400 according to the fourth embodiment of the presentinvention; and

FIGS. 9A to 9E are process sectional views showing a method ofmanufacturing the photoelectric conversion device 400 according to thefourth embodiment of the present invention.

DESCRIPTION OF THE EMBODIMENTS

A schematic structure of a photoelectric conversion device 100 accordingto the first embodiment of the present invention will be described belowwith reference to FIGS. 1 and 2. FIG. 1 is a schematic view showing thestructure of the photoelectric conversion device 100 according to thefirst embodiment of the present invention. FIG. 2 is a circuit diagramshowing the arrangement of a pixel according to the first embodiment ofthe present invention.

The photoelectric conversion device 100 includes a pixel array PA,vertical scanning circuit 10, and output circuit 20.

In the pixel array PA, a plurality of pixels P11 to Pmn aretwo-dimensionally laid out. Each of the pixels P11 to Pmn includes aphotoelectric conversion unit 1, first transfer unit 2, charge holdingportion 3, second transfer unit 4, charge-voltage converter 5, resetunit 6, output unit 7, and selection unit 8, as shown in FIG. 2.

The photoelectric conversion unit 1 executes a charge accumulationoperation for generating and accumulating a charge according to lightafter a first reset operation (to be described later) is completed. Thephotoelectric conversion unit 1 is, for example, a photodiode.

The first transfer unit 2 transfers a charge of the photoelectricconversion unit 1 to the charge holding portion 3. The first transferunit 2 is, for example, a transfer transistor, which transfers a chargeof the photoelectric conversion unit 1 to the charge holding portion 3when it is enabled in response to a transfer control signal φTX1 ofactive level supplied from the vertical scanning circuit 10 to its gate.

The charge holding portion 3 temporarily holds a charge which isgenerated by the photoelectric conversion unit 1 and is transferred bythe first transfer unit 2.

The second transfer unit 4 transfers a charge of the charge holding unit3 to the charge-voltage converter 5. The second transfer unit 4 is, forexample, a transfer transistor, which transfers a charge of the chargeholding unit 3 to the charge-voltage converter 5 when it is enabled inresponse to a transfer control signal φTX2 of active level supplied fromthe vertical scanning circuit 10 to its gate.

The charge-voltage converter 5 converts a charge transferred by thesecond transfer unit 4 into a voltage. The charge-voltage converter 5is, for example, a floating diffusion.

The reset unit 6 executes a first reset operation for resetting thephotoelectric conversion unit 1, charge holding portion 3, andcharge-voltage converter 5 while both the first and second transferunits 2 and 4 are ON. The reset unit 6 executes a second reset operationfor resetting the charge-voltage converter 5 while both the first andsecond transfer units 2 and 4 are OFF. The reset unit 6 is, for example,a reset transistor, which executes the aforementioned first or secondreset operation when it is enabled in response to a reset control signalφRES of active level supplied from the vertical scanning circuit 10 toits gate.

The output unit 7 outputs a signal corresponding to the voltage of thecharge-voltage converter 5 onto a signal line SL. The output unit 7 is,for example, an amplification transistor, which executes asource-follower operation together with a current source load 9connected to the signal line SL, thereby outputting a signalcorresponding to the voltage of the charge-voltage converter 5 onto thesignal line SL. That is, the output unit 7 outputs a noise signalcorresponding to the voltage of the charge-voltage converter 5 onto thesignal line SL in a state in which the charge-voltage converter 5 isreset by the reset unit 6 (i.e., the second reset operation iscomplete). The output unit 7 outputs a light signal corresponding to thevoltage of the charge-voltage converter 5 onto the signal line SL in astate in which a charge of the charge holding unit 3 is transferred bythe second transfer unit 4 to the charge-voltage converter 5.

The selection unit 8 sets the pixel in a selection/non-selection state.The selection unit 8 is, for example, a selection transistor, which setsthe pixel in a selection state when it is enabled in response to aselection control signal SEL of active level supplied from the verticalscanning circuit 10 to its gate. The selection unit 8 sets the pixel ina non-selection state when it is disabled in response to the selectioncontrol signal φSEL of non-active level supplied from the verticalscanning circuit 10 to its gate.

The vertical scanning circuit 10 drives the pixel array PA to controlthe respective pixels to execute the charge accumulation operations.More specifically, the vertical scanning circuit 10 simultaneouslycompletes the first reset operations of all the pixels in the pixelarray PA, thereby simultaneously starting the charge accumulationoperations by the photoelectric conversion units among all the pixels.Also, the vertical scanning circuit 10 simultaneously executes firsttransfer operations for enabling the first transfer units 2 while thesecond transfer units 4 are disabled in all the pixels of the pixelarray PA, thereby simultaneously completing the charge accumulationoperations by the photoelectric conversion units among all the pixels.In this way, a global electronic shutter function is implemented.

The vertical scanning circuit 10 selects a read row from which signalsare to be read out in the pixel array PA by scanning the pixel array PAin the vertical direction. The vertical scanning circuit 10 executessecond transfer operations for enabling the second transfer units 4while the first transfer units 2 are disabled in pixels in the selectedread row. Then, charges held by the charge holding portions 3 aretransferred to the charge-voltage converters 5 and are converted intovoltages, and the output units 7 output light signals according to thevoltages of the charge-voltage converters 5 onto the signal line SL inthe pixels in the read row. The vertical scanning circuit 10 executesand completes the second reset operations at different timings. Thus, inthe pixels in the read row, the output units 7 output noise signalsaccording to the voltages of the charge-voltage converters 5 onto thesignal line SL. That is, the vertical scanning circuit 10 sequentiallyexecutes the second transfer operations and second reset operations inpixels in respective rows of the pixel array PA, thereby controlling toexecute read operations of signals from the pixels of the respectiverows.

The output circuit 20 processes signals output from the pixels in theread row in the pixel array PA, and outputs the processed signals. Morespecifically, the output circuit 20 generates and outputs an imagesignal by calculating the differences between the noise signals andlight signals output from the pixels in the read row.

A layout arrangement example of the photoelectric conversion device 100according to the first embodiment of the present invention will bedescribed below with reference to FIG. 3. FIG. 3 is a view showing alayout arrangement example for four pixels of the photoelectricconversion device 100 according to the first embodiment of the presentinvention. The layout arrangement of a pixel P21 (see FIG. 1) bounded bythe one-dashed chain line in FIG. 3 will be mainly explained below, butthe layout arrangements of other pixels are the same as that of thepixel P21. Note that FIG. 3 corresponds to a view that sees throughrespective elements from a direction perpendicular to the upper surfaceof a semiconductor substrate SB (see FIGS. 4A and 4B). In FIG. 3,elements which are not required for a description are not illustrated.

In the pixel P21, a first transfer electrode 21 as the gate of the firsttransfer unit (transfer transistor) 2 is arranged at an upperneighboring position of the photoelectric conversion unit in FIG. 3. Tothe first transfer electrode 21, the vertical scanning circuit 10 (seeFIG. 1) supplies the first transfer control signal φTX1 via a firsttransfer control line 111.

The first transfer control line 111 extends in the right-and-leftdirection in FIG. 3 so as to cross the first transfer electrode 21. Thefirst transfer control line 111 is electrically connected to the firsttransfer electrode 21 via a contact plug 115 at a position where itcrosses the first transfer electrode 21.

The charge holding portion 3 is arranged at a position above thephotoelectric conversion unit 1 in FIG. 3 and in a region included in aregion where the first transfer electrode 21 is arranged, as indicatedby the one-dashed chain line. The first transfer electrode 21 extends tocover the charge holding portion 3 when viewed from the directionperpendicular to the upper surface of the semiconductor substrate SB.

A second transfer electrode 41 as the gate of the second transfer unit(transfer transistor) 4 is arranged at a right neighboring position ofthe first transfer electrode 21 and charge holding portion 3 in FIG. 3.To the second transfer electrode 41, the vertical scanning circuit 10(see FIG. 1) supplies the second transfer control signal φTX2 via asecond transfer control line 112.

The second transfer control line 112 extends in the right-and-leftdirection in FIG. 3 to cross the second transfer electrode 41. Thesecond transfer control line 112 is electrically connected to the secondtransfer electrode 41 via a contact plug 116 at a position where itcrosses the second transfer electrode 41.

The charge-voltage converter 5 is arranged at a right neighboringposition of the second transfer electrode 41 in FIG. 3. Thecharge-voltage converter 5 extends in an inverted L-shape.

A gate 61 of the reset unit (reset transistor) 6 is arranged at an upperneighboring position of the charge-voltage converter 5 in FIG. 3. To thegate 61, the vertical scanning circuit 10 (see FIG. 1) supplies thereset control signal (RES via a reset control line 113.

The reset control line 113 extends in the right-and-left direction inFIG. 3 to cross the gate 61. The reset control line 113 is electricallyconnected to the gate 61 via a contact plug 117 at a position where itcrosses the gate 61.

A gate 71 of the output unit (amplification transistor) 7 is arranged ata position below the charge-voltage converter 5 in FIG. 3. The gate 71is electrically connected to the charge-voltage converter 5 via a line(not shown). The gate 71 receives a voltage from the charge-voltageconverter 5.

A gate 81 of the selection unit (selection transistor) 8 is arranged ata position below the gate 71 in FIG. 3. To the gate 81, the verticalscanning circuit 10 (see FIG. 1) supplies the selection control signalφSEL via a selection control line 114.

The selection control line 114 extends in the right-and-left directionin FIG. 3 and extends to cross the gate 81. The selection control line114 is electrically connected to the gate 81 via a contact plug 118 at aposition where it crosses the gate 81.

In this way, by connecting the first and second transfer electrodes 21and 41 to different lines (first and second transfer control lines 111and 112) via the electrically isolated contact plugs, their potentialscan be independently controlled.

The sectional structure of the photoelectric conversion device 100according to the first embodiment of the present invention will bedescribed below with reference to FIGS. 4A and 4B. FIGS. 4A and 4B aresectional views showing the sectional structures of the photoelectricconversion device 100 according to the first embodiment of the presentinvention. FIG. 4A is a sectional view taken along a line A-B-C in FIG.3, and FIG. 4B is a sectional view taken along a line D-B in FIG. 3. Acase will be exemplified below wherein a carrier is an electron, and acharge carried by the carrier is a negative charge. However, the presentinvention is similarly applicable to a case in which a carrier is ahole, and a charge carried by the carrier is a positive charge, if n-and p-types replace each other.

As shown in FIG. 4B, the photoelectric conversion unit 1 is arranged ina p-type well 30 in the semiconductor substrate SB. The photoelectricconversion unit 1 includes a charge accumulation region 1 a used toaccumulate a charge, and a protection layer 1 b used to protect thecharge accumulation region 1 a. The charge accumulation region 1 acontains a heavily doped n-type impurity. The protection layer 1 bcontains a heavily doped p-type impurity.

As shown in FIG. 4B, a channel region 22 of the first transfer unit(transfer transistor) 2 is arranged between the charge accumulationregion 1 a and charge holding portion 3 in the p-type well 30 in thesemiconductor substrate SB. The channel region 22 is a semiconductorregion containing a p-type impurity. When the first transfer controlsignal of active level is supplied to the first transfer electrode 21via the first transfer control line 111 and contact plug 115, a channelwhich electrically connects the charge accumulation region 1 a andcharge holding portion 3 is formed in the channel region 22. When thefirst transfer control signal of non-active level is supplied to thefirst transfer electrode 21, the channel region 22 electricallydisconnects the charge accumulation region 1 a from the charge holdingportion 3.

The first transfer electrode 21 includes a first layer 21 a and secondlayer 21 b. The first layer 21 a extends along an upper surface SBa ofthe semiconductor substrate SB. The second layer 21 b is arranged on thefirst layer 21 a, and has a lower transmittance of light than the firstlayer 21 a. The first layer 21 a is formed of, for example, polysilicon.The second layer 21 b is formed of, for example, a metal silicidematerial. The second layer 21 b is formed of, for example, a materialprepared by silicidating a refractory metal such as cobalt, tungsten,nickel, or titanium. When the first layer 21 a is formed of polysilicon,and the second layer 21 b is formed of a metal silicide, the secondlayer 21 b has a lower transmittance of light than the first layer 21 a.

On the other hand, as shown in FIG. 4A, a channel region 42 of thesecond transfer unit (transfer transistor) 4 is arranged between thecharge holding portion 3 and charge-voltage converter 5 in the p-typewell 30 in the semiconductor substrate SB. The channel region 42 is asemiconductor region containing a p-type impurity. When the secondtransfer control signal of active level is supplied to the secondtransfer electrode 41 via the second transfer control line 112 andcontact plug 116 (see FIG. 3), a channel which electrically connects thecharge holding portion 3 and charge-voltage converter 5 is formed in thechannel region 42. When the second transfer control signal of inactivelevel is supplied to the second transfer electrode 41, the channelregion 42 electrically disconnects the charge holding portion 3 from thecharge-voltage converter 5.

The second transfer electrode 41 is formed of the same material as thatof the first layer 21 a in the first transfer electrode 21. The secondtransfer electrode 41 is formed of, for example, polysilicon. The secondtransfer electrode 41 undergoes potential control independently of thefirst transfer electrode 21.

A case will be examined below in which the second transfer electrode isarranged to completely cover the first transfer electrode. In this case,in order to supply the first transfer control signal, which is differentfrom the second transfer control signal supplied to the second transferelectrode, to the first transfer electrode, an opening has to be formedin the second transfer electrode to partially expose the upper surfaceof the first transfer electrode. Then, a contact plug has to be formedto have a size smaller than that opening diameter, and to be connectedto the first transfer electrode. As a result, a layer structureincluding the first and second transfer electrodes and the contact plugsrequired to supply control signals to these electrodes becomescomplicated.

Also, in this case, the second transfer electrode extends up to a regionabove the light-receiving surface of the photoelectric conversion unitso as to completely cover the first transfer electrode. Then, anunintended potential barrier may be formed in the photoelectricconversion unit at a timing at which a charge is to be transferred fromthe photoelectric conversion unit to the charge holding portion, and thecharge transfer efficiency from the photoelectric conversion unit to thecharge holding portion may deteriorate.

By contrast, in this embodiment, the first transfer electrode 21 extendsto cover the charge holding portion 3 to optically shield the chargeholding portion 3, and not to overlap the second transfer electrode whenviewed from the direction perpendicular to the upper surface of thesemiconductor substrate SB. Thus, a layer structure including the firstand second transfer electrodes, and contact plugs required to supplycontrol signals to these electrodes can be simplified. Since the secondtransfer electrode does not extend up to a region above thelight-receiving surface of the photoelectric conversion unit, the chargetransfer efficiency from the photoelectric conversion unit to the chargeholding portion is hard to deteriorate.

Furthermore, in this embodiment, the first transfer electrode 21includes the first layer 21 a and the second layer 21 b which has alower transmittance of light than the first layer 21 a. That is, thefirst transfer electrode 21 itself has light-shielding performance.Then, light beams PB1 and PB2 which become incident in the vicinity ofthe charge holding portion 3 are easily reflected or absorbed by thesurface of the second layer 21 b, that is, an upper surface 21 c of thefirst transfer electrode 21. In this way, since incidence of light tothe charge holding portion 3 is suppressed, the charge holding portion 3can be sufficiently optically shielded even when the second transferelectrode is arranged not to completely cover the first transferelectrode.

Note that the incoming light beam PB2 becomes incident on an elementisolation region under the first transfer electrode but it does notdirectly enter the charge holding portion if the first transferelectrode 21 itself does not have any light-shielding performance.However, the light beam which becomes incident on the element isolationregion becomes stray light components, and some components may leak intothe charge holding portion. Also, a charge generated at the lowerportion of the element isolation region may be mixed into the chargeholding portion. In order to obtain performance equal to thelight-shielding performance of a mechanical shutter by the globalelectronic shutter function, it is important to suppress such slightleak components and mixed components.

Also, the upper surface 21 c of the first transfer electrode 21 and anupper surface 41 c of the second transfer electrode 41 have nearly equalheights from the upper surface SBa of the semiconductor substrate SB. Aheight H1 of the upper surface 21 c of the first transfer electrode 21is nearly equal to a height H2 of the upper surface 41 c of the secondtransfer electrode 41. The first and second transfer electrodes 21 and41 have bottom surfaces having nearly equal heights. That is, the firstand second transfer electrodes 21 and 41 are arranged at the same level.This is because a polysilicon layer is formed to simultaneously form thefirst and second transfer electrodes 21 and 41 in a single process. Thatis, this polysilicon layer is patterned to obtain patterns of the firstand second transfer electrodes 21 and 41, and the pattern of the firsttransfer electrode 21 selectively undergoes metal silicidation. In thismanner, the first and second transfer electrodes 21 and 41 can besimultaneously formed. That is, a structure in which the first andsecond transfer electrodes are arranged at the same level is suited toformation in a simple process. Note that the polysilicon layer may bepatterned in either a single process or different processes.

Therefore, according to this embodiment, there can be provided aphotoelectric conversion device which is advantageous to simplify aprocess for forming a structure required to optically shield the chargeholding portion which temporarily holds a charge generated by thephotoelectric conversion unit. Since the first and second transferelectrodes are arranged at the same level, a flat structure can beeasily obtained without forming any thick insulation film on the upperportions of the first and second transfer electrodes.

A buried channel region may be formed between the charge accumulationregion 1 a and charge holding portion 3 shown in FIG. 4B in place of thechannel region 22. This buried channel region is an n-type semiconductorregion formed by implanting an n-type impurity into a region having apredetermined depth from the surface SBa of the semiconductor substrateSB. The buried channel is a channel having a potential that makes acharge which overflows from the charge accumulation region 1 a flow intothe charge holding portion 3 during the time period in which the chargeaccumulation region 1 a accumulates the charge. In this case, since acharge that overflows from the charge accumulation region 1 a can beused, a wider dynamic range can be assured. Since a time period in whicha charge generated by the photoelectric conversion unit 1 is held in thecharge holding portion 3 becomes longer, sufficiently highlight-shielding performance is required. In order to obtain suchsufficiently high light-shielding performance, the structure in whichthe first transfer electrode itself has light-shielding performance isadvantageous.

Alternatively, the effects of the present invention can also be obtainedwhen the first transfer electrode is formed as a so-called metal gateelectrode so that the first transfer electrode itself haslight-shielding performance.

FIG. 5 shows an example of an image capturing system to which thephotoelectric conversion device of the present invention is applied.

An image capturing system 90 mainly includes an optical system, an imagecapturing apparatus 86, and a signal processing unit, as shown in FIG.5. The optical system mainly includes a shutter 91, imaging lens 92, andstop 93. The image capturing apparatus 86 includes the photoelectricconversion device 100. The signal processing unit mainly includes acaptured signal processing circuit 95, A/D converter 96, image signalprocessor 97, memory unit 87, external I/F unit 89, timing generator 98,overall control/arithmetic unit 99, recording medium 88, and recordingmedium control I/F unit 94. Note that the signal processing unit may notinclude the recording medium 88.

The shutter 91 is arranged in front of the imaging lens 92 on an opticalpath, and controls exposure.

The imaging lens 92 refracts incoming light, and forms an object imageon an imaging surface of the photoelectric conversion device 100 of theimage capturing apparatus 86.

The stop 93 is arranged between the imaging lens 92 and photoelectricconversion device 100 on the optical path, and adjusts the amount oflight guided to the photoelectric conversion device 100 after the lighthas passed through the imaging lens 92.

The photoelectric conversion device 100 of the image capturing apparatus86 converts an object image formed on its imaging surface into an imagesignal. The image capturing apparatus 86 reads out the image signal fromthe photoelectric conversion device 100 and outputs the readout imagesignal.

The captured signal processing circuit 95 is connected to the imagecapturing apparatus 86, and processes the image signal output from theimage capturing apparatus 86.

The A/D converter 96 is connected to the captured signal processingcircuit 95, and converts the image signal (analog signal) afterprocessing output from the captured signal processing circuit 95 into animage signal (digital signal).

The image signal processor 97 is connected to the A/D converter 96, andgenerates image data by applying arithmetic processing such as variouscorrections to the image signal (digital signal) output from the A/Dconverter 96. This image data is supplied to the memory unit 87,external I/F unit 89, overall control/arithmetic unit 99, recordingmedium I/F unit 94, and the like.

The memory unit 87 is connected to the image signal processor 97, andstores the image data output from the image signal processor 97.

The external I/F unit 89 is connected to the image signal processor 97.Then, the image data output from the image signal processor 97 istransferred to an external apparatus (e.g., a personal computer) via theexternal I/F unit 89.

The timing generator 98 is connected to the image capturing apparatus86, captured signal processing circuit 95, A/D converter 96, and imagesignal processor 97. The timing generator 98 supplies timing signals tothe image capturing apparatus 86, captured signal processing circuit 95,A/D converter 96, and image signal processor 97. Then, the imagecapturing apparatus 86, captured signal processing circuit 95, A/Dconverter 96, and image signal processor 97 operate in synchronism withthe timing signals.

The overall control/arithmetic unit 99 is connected to the timinggenerator 98, image signal processor 97, and recording medium controlI/F unit 94, and totally controls the timing generator 98, image signalprocessor 97, and recording medium control I/F unit 94.

The recording medium 88 is detachably connected to the recording mediumcontrol I/F unit 94. The image data output from the image signalprocessor 97 is recorded on the recording medium 88 via the recordingmedium control I/F unit 94.

With the above arrangement, when a satisfactory image signal is obtainedby the photoelectric conversion device 100, a satisfactory image (imagedata) can be obtained.

A photoelectric conversion device 200 according to the second embodimentof the present invention will be described below with reference to FIG.6. FIG. 6 is a sectional view showing the sectional structure of thephotoelectric conversion device 200 according to the second embodimentof the present invention. Differences from the first embodiment will bemainly explained below.

In the photoelectric conversion device 200, the structure of a secondtransfer electrode 241 is different from the first embodiment. Thesecond transfer electrode 241 includes a third layer 241 a and fourthlayer 241 b. The third layer 241 a has the same height from an uppersurface SBa of a semiconductor substrate SB as that of a first layer 21a in a first transfer electrode 21, and extends along the upper surfaceSBa of the semiconductor substrate SB. The fourth layer 241 b has thesame height from the upper surface SBa of the semiconductor substrate SBas that of a second layer 21 b in the first transfer electrode 21, isarranged on the third layer 241 a, and has a lower transmittance oflight than the third layer 241 a. The third layer 241 a is formed of thesame material as the first layer 21 a, for example, polysilicon. Thefourth layer 241 b is formed of the same material as the second layer 21b, for example, a metal silicide material. The fourth layer 241 b isformed of, for example, a material prepared by siliciding a refractorymetal such as cobalt, tungsten, nickel, or titanium. When the thirdlayer 241 a is formed of polysilicon, and the fourth layer 241 b isformed of a metal silicide, the fourth layer 241 b has a lowertransmittance of light than the third layer 241 a.

According to the structure of this embodiment, since light that becomesincident on the second transfer electrode 241 is reflected or absorbedby its upper surface 241 c, light leakage into a charge holding portion3 can be further suppressed.

Also, a polysilicon layer used to form the first and second transferelectrodes 21 and 241 can be simultaneously formed in a single process.Then, after the polysilicon layer is patterned to obtain patterns of thefirst and second transfer electrodes 21 and 241, the pattern of thesecond transfer electrode 241 selectively undergoes metal silicidationin addition to that of the first transfer electrode 21, therebysimultaneously forming the first and second transfer electrodes 21 and241. That is, the structure of this embodiment is suited to formation ina simple process. Therefore, this embodiment can also provide thephotoelectric conversion device which is advantageous to simplify aprocess for forming a structure required to optically shield the chargeholding portion which temporarily holds a charge generated by aphotoelectric conversion unit.

FIGS. 7A and 7B show a photoelectric conversion device 300 according tothe third embodiment of the present invention. FIGS. 7A and 7B are viewsshowing the structure of the photoelectric conversion device 300according to the third embodiment of the present invention. FIG. 7Ashows a layout arrangement example of the photoelectric conversiondevice 300, and FIG. 7B shows the sectional structure of thephotoelectric conversion device 300. Differences from the first andsecond embodiments will be mainly explained below.

Depending on performance required to implement the global electronicshutter function, even when the upper portions of first and secondtransfer electrodes are formed of a metal silicide, a charge holdingportion may not often be sufficiently optically shielded.

By contrast, in this embodiment, in the photoelectric conversion device300, an insulation film 360 and light-shielding film 350 are furtherarranged. The insulation film 360 extends to cover first and secondtransfer electrodes 21 and 241. The light-shielding film 350 is arrangedon the first transfer electrode 21 via the insulation film 360. Thelight-shielding film 350 extends up to a region above the secondtransfer electrode 241 via the insulation film 360. A material of thelight-shielding film 350 includes those having high light-shieldingperformance such as tungsten, tungsten silicide, and aluminum.

Also, in this embodiment, a predetermined gap is assured so as toprevent a contact plug 115 and the light-shielding film 350 from beingelectrically connected (see FIG. 7A). Likewise, a gap is assured toprevent a contact plug 116 and the light-shielding film 350 from beingelectrically connected (see FIG. 7A). That is, the first and secondtransfer electrodes can independently undergo potential control.

When light becomes incident in the vicinity of the contact plugs 115 and116, a light-shielding effect by the light-shielding film 350 cannot beexpected. However, since the upper portions (metal silicide) of thefirst and second transfer electrodes have light-shielding performance,leakage of light into the charge holding portion can be prevented.

In FIGS. 7A and 7B, the potential of the light-shielding film 350 is notfixed, that is, in a floating state. However, the present invention isnot limited to this, and the potential of the light-shielding film 350may be configured to be controllable.

FIGS. 8A and 8B show a photoelectric conversion device 400 according tothe fourth embodiment of the present invention. FIGS. 8A and 8B areviews showing the structure of the photoelectric conversion device 400according to the fourth embodiment of the present invention. FIG. 8Ashows a layout arrangement example of the photoelectric conversiondevice 400, and FIG. 8B shows the sectional structure of thephotoelectric conversion device 400. Differences from the first to thirdembodiments will be mainly explained below.

In the photoelectric conversion device 400, a light-shielding film 450is connected to a first transfer control line 111 via a contact plug415, and also to a first transfer electrode 21 via a contact plug 451.Then, the light-shielding film 450 can also shield light in the vicinityof the contact plug 451 connected to the first transfer electrode 21.That is, since the light-shielding film 450 can be laid out on a broaderregion on the first transfer electrode, leakage of light into the chargeholding portion can be further suppressed. Also, the driving performanceof the first transfer electrode 21 can be further improved.

Note that a contact plug 116 and the light-shielding film 450 areelectrically isolated as in the third embodiment. In this way, thepotentials of the first and second transfer electrodes can beindependently controlled.

In this embodiment, the first transfer electrode and light-shieldingfilm are connected, and the second transfer electrode andlight-shielding film are isolated. However, the present invention is notlimited to such specific structure. That is, the light-shielding filmmay be electrically connected to one of the first and second transferelectrodes, and may be electrically isolated from the other. When thesecond transfer electrode and light-shielding film are connected, andthe first transfer electrode and light-shielding film are isolated, thatlight-shielding film can also shield light in the vicinity of thecontact plug connected to the second transfer electrode. That is, sincethe light-shielding film can be laid out on a broader region on thesecond transfer electrode, leakage of light into the charge holdingportion can be further suppressed. Whether the light-shielding film isto be connected to the first or second transfer electrode may beselected to further improve the light-shielding performance inconsideration of a layout inside a pixel. Also, the driving performanceof the transfer electrode connected to the light-shielding film can beimproved.

A method of manufacturing the photoelectric conversion device 400according to the fourth embodiment of the present invention will bedescribed below with reference to FIGS. 9A to 9E. FIGS. 9A to 9E areprocess sectional views showing the method of manufacturing thephotoelectric conversion device 400 according to the fourth embodimentof the present invention. In each of FIGS. 9A to 9E, a left-side viewcorresponds to a section taken along a D1-B1 line in FIG. 8A, and aright-side view corresponds to a section taken along a line A1-B1-C1line in FIG. 8A. Note that a peripheral circuit portion region is notillustrated.

In a process shown in FIG. 9A, an n-type well (not shown) and a p-typewell 30 are formed in a semiconductor substrate SB, and an STI or LOCOSelement isolation portion 301 is then formed. The semiconductorsubstrate SB is formed of, for example, silicon.

Next, an n-type impurity is doped (by ion implantation) in thesemiconductor substrate SB using a predetermined resist pattern as amask, thereby forming a charge holding portion 3. Furthermore, variousimpurities may be implanted as needed to provide an electronic shutterfunction.

Subsequently, first and second polysilicon layers 103 and 104 aresimultaneously formed on a prospective formation region of a firsttransfer electrode 21 and that of a second transfer electrode 241 on thesemiconductor substrate SB. In this process (first process), the firstpolysilicon layer 103 is formed to cover the charge holding portion 3and not to overlap the second polysilicon layer 104 when viewed from thedirection perpendicular to an upper surface SBa of the semiconductorsubstrate SB.

After that, an n-type impurity is doped to form a charge accumulationregion 1 a of a photoelectric conversion unit 1. Next, a p-type impurityis doped to form a protection layer 1 b required to form a buriedstructure of the photoelectric conversion unit (photodiode) 1. Thecharge accumulation region 1 a is formed in the semiconductor substrateSB deeper than the charge holding portion 3.

An n-type impurity is then doped by ion implantation using a gateelectrode as a mask, so as to form a semiconductor region, for example,a charge-voltage converter 5, that configures a part of a lightly dopedsource or drain self-aligned to the side surface of the gate electrode.Furthermore, a process for implanting various impurities may be executedin this case to provide an electronic shutter function.

In a process (second process) shown in FIG. 9B, an insulation film 303is formed to cover the first and second polysilicon layers 103 and 104.The insulation film 303 is formed to have a stacked structure of, forexample, a silicon nitride film and silicon oxide film. In this case,the silicon nitride film can be used as an anti-reflection film for alight-receiving surface of the photoelectric conversion unit 1 or aliner film of a self-align contact. However, the present invention isnot limited to this. The insulation film 303 is formed to at least coverthe photoelectric conversion unit 1 in addition to the first and secondpolysilicon layers 103 and 104. With this structure, the photoelectricconversion unit 1 is protected from any etching damage in an etchingprocess later.

The insulation film 303 is patterned to form a silicide protection. Aresist pattern RP1 which selectively covers a non-silicide regionincluding the photoelectric conversion unit 1 is formed. Subsequently(in a third process), the insulation film 303 is etched using the resistpattern RP1 as a mask to expose the upper surfaces of the first andsecond polysilicon layers 103 and 104. At this time, the surfaces of thesource and drain of a transistor in a pixel may be exposed. Note thatthe boundary of the resist pattern RP1 can be set to be closer to thecenter side of the photoelectric conversion unit 1 than the boundarybetween the photoelectric conversion unit 1 and first transfer electrode21. Since the insulation film 303 is also formed on the side wall of thefirst polysilicon layer 103, even when the boundary of the resistpattern RP1 is set on the center side of the photoelectric conversionunit 1, the photoelectric conversion unit 1 is prevented from beingsilicidated in a silicidation process to be described later, and thesurface of the photoelectric conversion unit 1 is unlikely to be exposedby dry etching in the next process. When an interval between the firstand second polysilicon layers 103 and 104 is as small as, for example,about 0.2 μm, a gap portion which forms a gap between these layers ishardly silicidated even when no resist pattern is formed. Hence, in thiscase, a resist pattern may not be formed on the gap portion between thefirst and second polysilicon layers 103 and 104.

In the sequence of this manufacturing method, the resist pattern RP1also covers, for example, a non-silicide region such as an active regionof a transistor in a pixel. Using this resist pattern RP1, the structureof a transistor in a peripheral circuit portion and that in a pixel maybe changed. For example, a transistor with an LDD structure having asource and drain formed of lightly and heavily doped regions may beformed in the peripheral circuit portion, and a transistor formed ofonly lightly doped regions may be formed in a pixel. This structure canbe obtained by forming a heavily doped region of the peripheral circuitportion using the resist pattern RP1 as a mask. In this way, a merit ofshortening the process can be expected.

When a transistor in a pixel is configured to also have an LDDstructure, after protection of the photoelectric conversion unit 1 andformation of the LDD structure of a transistor are made again usinganother mask, the resist pattern RP1 may be formed to form a silicideprotection. Note that silicidation of the surfaces of the first andsecond transfer electrodes in a pixel aims at enhancing thelight-shielding performance, and that of the peripheral circuit portionmay also be made in the same process. Note that the silicide protectionmay also serve as an anti-reflection film or liner film, as describedabove.

Subsequently (in a fourth process), a metal layer (not shown) is formedto cover the first polysilicon layer 103, second polysilicon layer 104,and etched insulation film 303. The metal layer is formed of arefractory metal such as cobalt, tungsten, nickel, or titanium. Afterthat (a fifth process, silicidation process), annealing is performed tosilicidate the upper portions of the first and second polysilicon layers103 and 104, respectively. That is, the upper portions of the first andsecond polysilicon layers 103 and 104 are converted to a second layer(first metal silicide layer) 21 b and fourth layer (second metalsilicide layer) 241 b. Then, the first transfer electrode 21 includingthe first layer (the lower portion of the first polysilicon layer 103)21 a and the second layer (first metal silicide layer) 21 b is formed.At the same time, the second transfer electrode 241 including a thirdlayer (the lower portion of the second polysilicon layer) 241 a and thefourth layer (second metal silicide layer) 241 b is formed. That is, thefirst and second transfer electrodes 21 and 241 are formedsimultaneously.

In the process shown in FIG. 9C, an unreacted refractory metal in themetal layer is removed by etching off. Although the peripheral circuitportion region is not illustrated, polysilicon and an active region ofthe peripheral circuit portion may be simultaneously silicidated asneeded.

In the process shown in FIG. 9D, an insulation film 360 is formed tocover the first and second transfer electrodes 21 and 241. After that, acontact hole is formed in the insulation film 360, and a metal such astungsten is buried in that contact hole, thus forming a contact plug451. Then, a light-shielding film 450 is formed to cover the uppersurface of the contact plug 451 and to extend from a region above thefirst transfer electrode 21 to that above the second transfer electrode241 via the insulation film 360. The light-shielding film 450 is formedof, for example, a material having high light-shielding performance suchas tungsten, tungsten silicide, or aluminum.

Note that the contact plug 451 and light-shielding film 450 may besimultaneously formed using the same material. Then, the formationprocess of the contact plug 451 and light-shielding film 450 can besimplified. The light-shielding film 450 may be formed by burying amaterial in grooves patterned on the insulation film in place ofpatterning of the metal layer.

In the process shown in FIG. 9E, an interlayer insulation film 416 isformed to cover the light-shielding film 450. The interlayer insulationfilm 416 uses, for example, a BPSG film. Then, a contact hole is formedat a predetermined position, and a metal such as tungsten is buried inthat contact hole, thereby forming a contact plug 415. After that, awiring pattern 110 including first and second transfer control lines 111and 112 is formed.

Furthermore, although not shown, other interlayer insulation films,other interconnection layers, an intra-layer lens, color filters,microlens, and the like are formed, but a description thereof will notbe given.

When a semi-global electronic shutter function is implemented using aphotoelectric conversion device, lines have to be connected to thesource, drain, and gate of each transistor in a pixel via contact plugs.Also, the gate of each transistor in a pixel has to independentlyundergo potential control. In such photoelectric conversion device, whenthe light-shielding film 450 is arranged, gaps have to be formed betweenthe respective contact plugs and light-shielding film 450 so as toelectrically isolate them. It is ideal to completely cover regions otherthan the photoelectric conversion unit using a light-shielding film, butit is hard to implement due to the aforementioned problem. Hence, inthis embodiment, a silicide layer is formed on at least portions of thefirst transfer electrode, second transfer electrode, and an activeregion on which no light-shielding film is arranged. However, a floatingdiffusion serving as the charge-voltage converter may be configured notto form a silicide layer on its upper portion in consideration ofholding a charge.

Alternatively, a silicide layer may be formed on the upper portion ofthe charge-voltage converter. More specifically, the charge-voltageconverter 5 is formed to include first and second regions. The secondregion is arranged on the first region to form a part of the uppersurface of the semiconductor substrate SB, and is formed of a metalsilicide material. Thus, leakage of light that becomes incident on thecharge-voltage converter 5 into the charge holding portion can bereduced.

A gate 61 of a reset unit (reset transistor) 6, a gate 71 of an outputunit (amplification transistor) 7, a gate 81 of a selection unit(selection transistor) 8, and active regions of respective transistorsare partially silicidated. Then, leakage of light that becomes incidenton the silicide regions into the charge holding portion can be furtherreduced.

The structures of the aforementioned embodiments can be combined asneeded.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2009-005135, filed Jan. 13, 2009, which is hereby incorporated byreference herein in its entirety.

1. A photoelectric conversion device comprising: a photoelectricconversion unit which is arranged in a semiconductor substrate; a chargeholding portion which is arranged in the semiconductor substrate andtemporarily holds a charge generated by the photoelectric conversionunit; a first transfer electrode which is arranged at a position abovethe semiconductor substrate to transfer a charge generated by thephotoelectric conversion unit to the charge holding portion; acharge-voltage converter which is arranged in the semiconductorsubstrate and converts a charge into a voltage; and a second transferelectrode which is arranged at a position above the semiconductorsubstrate to transfer a charge held by the charge holding portion to thecharge-voltage converter, wherein the first transfer electrode isarranged to cover the charge holding portion so as to optically shieldthe charge holding portion, and not to overlap the second transferelectrode.
 2. The device according to claim 1, wherein the firsttransfer electrode includes: a first layer which extends along an uppersurface of the semiconductor substrate; and a second layer which isarranged above the first layer to optically shield the first layer. 3.The device according to claim 1, wherein the first transfer electrode isformed of a metal.
 4. The device according to claim 2, wherein thesecond layer is formed of a metal silicide material.
 5. The deviceaccording to claim 1, wherein the first transfer electrode and thesecond transfer electrode are arranged to have the same height from anupper surface of the semiconductor substrate.
 6. The device according toclaim 2, wherein the second transfer electrode includes: a third layerwhich has the same height from an upper surface of the semiconductorsubstrate as the first layer, and extends along the upper surface of thesemiconductor substrate; and a fourth layer which has the same heightfrom the upper surface of the semiconductor substrate as the secondlayer, and is arranged on the third layer to optically shield the thirdlayer.
 7. The device according to claim 6, wherein the fourth layer isformed of a metal silicide material.
 8. The device according to claim 4,wherein the charge-voltage converter includes: a first region; and asecond region which is arranged on the first region to form a portion ofthe upper surface of the semiconductor substrate, and is formed of ametal silicide material.
 9. The device according to claim 1, furthercomprising: an insulation film which extends to cover the first transferelectrode; and a light-shielding film which is arranged at a positionabove the first transfer electrode via the insulation film.
 10. Thedevice according to claim 9, wherein the insulation film extends tofurther cover the second transfer electrode, and the light-shieldingfilm extends up to a region above the second transfer electrode via theinsulation film.
 11. The device according to claim 9, wherein thelight-shielding film is electrically connected to one of the firsttransfer electrode and the second transfer electrode, and iselectrically disconnected from the other of the first transfer electrodeand the second transfer electrode.
 12. An image capturing systemcomprising: a photoelectric conversion device according to claim 1; anoptical system which forms an image on an imaging surface of thephotoelectric conversion device; and a signal processing unit whichgenerates image data by processing a signal output from thephotoelectric conversion device.
 13. A method of manufacturing aphotoelectric conversion device having a semiconductor substrate inwhich a photoelectric conversion unit, a charge holding portion whichtemporarily holds a charge generated by the photoelectric conversionunit, and a charge-voltage converter which converts a charge into avoltage are arranged, the method comprising: the first step ofsimultaneously executing formation of a first polysilicon layer on thesemiconductor substrate, of a first transfer electrode used to transfera charge generated by the photoelectric conversion unit to the chargeholding portion, and formation of a second polysilicon layer on thesemiconductor substrate, of a second transfer electrode used to transfera charge of the charge holding portion to the charge-voltage converter;the second step of forming an insulation film to cover the firstpolysilicon layer and the second polysilicon layer; the third step ofetching the insulation film to expose an upper surface of the firstpolysilicon layer and an upper surface of the second polysilicon layer;the fourth step of forming a metal layer to cover the first polysiliconlayer, the second polysilicon layer, and the etched insulation film; andthe fifth step of performing annealing to respectively convert an upperportion of the first polysilicon layer and an upper portion of thesecond polysilicon layer into a first metal silicide layer and a secondmetal silicide layer, so as to simultaneously form the first transferelectrode including a lower portion of the first polysilicon layer andthe first metal silicide layer, and the second transfer electrodeincluding a lower portion of the second polysilicon layer and the secondmetal silicide layer, wherein in the first step, the first polysiliconlayer is formed to cover the charge holding portion and not to overlapthe second polysilicon layer, and in the fifth step, the first transferelectrode is formed to cover the charge holding portion so as tooptically shield the charge holding portion, and not to overlap thesecond transfer electrode.
 14. The method according to claim 13, whereinin the second step, the insulation film is formed to further cover thephotoelectric conversion unit.